To better protect occupants in an automobile it is necessary to measure the belt force of a person wearing a seatbelt. The data is processed and used, during an accident for instance, to calculate the optimum triggering of belt tensioners and airbags.
A measuring device should measure belt forces in the range 0–150 N with an accuracy of approx. +/−2.5%. Belt forces of up to 10,000 N can occur during standard operation of the automobile's force-sensing device when a person is belted. These forces must be measured properly and must not cause damage to the measuring device. During an accident the belt force can rise to, and exceed, 20,000 N. The seatbelt lock or seatbelt lock casing may, in this case, sustain permanent deformation damage but must not fail to operate properly. The force-sensing device may be damaged in the course of this event.
A tried-and-tested principle for measuring forces is to measure displacement on a resilient component on which a force has impinged (spiral spring, torsion spring, coil spring). An overload can easily be intercepted here if, after traveling beyond a path or angle specified in the design, the resilient component strikes a mechanical stop (step, edge).
U.S. Pat. No. 6,230,088 B1 discloses a seatbelt lock casing which is connected via a force-sensing device to an anchoring point secured to a vehicle's bodywork. Such an arrangement is complicated to install and exhibits poor force flow.